As I have already anticipated last week, the differences in the central nervous system (CNS) among vertebrates was a very interesting course, so today I would like to talk about a byword for visual acuity, “hawk-eyed”, and why it is known that birds see better than humans.
We use the catchword “hawk-eyed” as an adjective for people having very good eyesight. Indeed, birds of prey such as kestrels are famous for their ability to spot tiny prey animals from high above the ground and, as one study by Fox et al. in 1976 found, the American kestrel can see a 2mm insect at a range of 18 meters.
Their legendary visual ability and the keenness of their eyesight is due to a variety of factors. One of them is the eyes size: birds have relatively large eyes compared to mammals and, in general, birds’ eyes are around twice the size (relative to body size) than those of mammals, accounting for 15% of the mass of the bird’s entire head. In simple terms, a bigger eye means better vision, and excellent vision is essential for avoiding collisions in flight or for capturing fast-moving or camouflaged prey. Indeed, it goes without saying that the birds with the largest eyes relative to body size are eagles, falcons (birds of prey) and owls (nocturnal birds).
Another factor that gives birds a pretty great eyesight is the number of photoreceptors in the retina. Both birds and humans have photoreceptive ‘cones’ in the retina located at the back of the eye. Cones respond differently to light of different wavelengths, allowing us to see color light. The human eye contains 10,000 cones per square millimeter. Songbirds, on the other hand, have up to 12 times this amount or 120,000 cones per square millimeter. Moreover, we, humans, have three types of cones, each one sensitive to red, green, or blue light. This is called trichromatic color vision. Compared with many mammals, humans and primates have relatively good color vision, because most others—such as dogs—have only two cone types. Birds have an extra cone for tetrachromatic color vision, expanding the visible light spectrum and allowing birds to see ultraviolet frequencies (see picture below).
In addition, birds’ cone cells contain a coloured oil droplet to improve distance vision especially in hazy conditions. The oil droplet acts as a filter, removing some wavelengths and narrowing the absorption spectra of the pigments. It basically reduces the response overlap between pigments and increases the number of colours that a bird can discern.
On the other hand, during low-light conditions, both humans and birds rely on photoreceptive ‘cell rods’ in the retina. These photoreceptors are more sensitive than cone cells and are almost entirely responsible for night vision. The human eye has 200,000 cell rods per square millimeter. Some birds, such as owls, have up to 1,000,000 cell rods per square millimeter. Besides, bird retinas (the innermost, light-sensitive layer of tissue of the eye), in contrast to humans, contain no blood vessels, preventing light scattering and thus providing birds with greater visual acuity than humans.
To sum up, the size of eyes is important precisely because the larger the eye, the larger the image on the retina. Just think about watching a 12-inch television screen compared with a 40-inch screen: bigger eyes have more light receptors in the same way that larger TV screens have more pixels, and hence a better image.
Apart from these anatomical and light perception differences, birds have some other pretty enthralling characteristics still somehow vision related that I would like to quickly talk about. The first of these features is that some birds (such as ducks, falcons, and gulls) sleep with one eye open and they share this feature with some marine mammals, which need to return to the surface to breathe. What is even more interesting, and in my opinion mind-blowing, is that when a bird is sleeping, for example, with its right eye open, it is resting the right brain hemisphere. This ability is actually incredibly useful. Just think about predators: ducks and gulls often sleep on the ground and are vulnerable to predators, so it pays to keep an eye open. Another circumstance in which this feature is incredibly useful is when bird sleep while flying. I am aware that this idea seems absurd, but apart from observations like the one by the ornithologist David Lack, that noticed that European swifts ascend into the sky at dusk and do not return until the following morning, glaucous-winged gulls in North America, have been seen flying to their roosts with only one eye open, suggesting that they are already sleeping before even reaching the roost.
The last feature I would like to mention involve the navigation with sun and stars. It was long known that birds use them to orientate and migrate across seemingly endless oceans and/or large land mass. However, it was recently discovered that birds possess an internal magnetic compass. It has been hypothesised that a chemical mechanism based in the eye provides the compass, allowing them to ‘see’ the earth’s magnetic fields, while at the same time magnetite (a magnetic mineral) receptors in the beak provide the map: the chemical mechanism, the compass, might detect the direction of the magnetic field, while the magnetite in the beak, the map, detects the strength of the magnetic field and by integrating both types of information the birds can find their way home. We currently have a good basic understanding of some of the senses of birds, but I believe that the best is yet to come.